Apparatuses and methods for physical broadcast channel (pbch) assisted synchronization during a discontinuous reception (drx) operation

ABSTRACT

A mobile communication device with a Radio Frequency (RF) unit and a processing unit is provided. The RF unit transmits and receives wireless signals to and from a service network. The processing unit configures the RF unit to communicate with the service network in a DRX operation, and uses a Physical Broadcast Channel (PBCH) for synchronizing with the service network during the DRX operation.

CROSS REFERENCE TO RELATED APPLICATIONS

This Application claims priority of U.S. Provisional Application No.61/921,634, filed on Dec. 30, 2013, the entirety of which isincorporated by reference herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention generally relates to synchronization of mobilecommunications, and more particularly, to apparatuses and methods forPhysical Broadcast Channel (PBCH) assisted synchronization during aDiscontinuous Reception (DRX) operation.

2. Description of the Related Art

In a typical mobile communication environment, communications between amobile device and a service network are performed arbitrarily accordingto the requests from or to the user of the mobile device. Since wirelesstransmission and reception are not necessarily performed at all times,keeping the mobile device always operating in transmission mode orreception mode causes unnecessary power consumption. For conservingbattery power of the mobile device and saving wireless resources of theservice network, a technique called Discontinuous Reception (DRX)operation is employed when there is no potential wireless transmissionand reception for the mobile device in a forthcoming period of time. Asshown in FIG. 1, in the OFF duration of the DRX operation, the servicenetwork does not page the mobile device and the mobile device enters asleep mode in which at least the functionality of wireless transmissionand reception is turned off. In the ON duration of the DRX operation,the mobile device wakes up from the sleep mode and turns on thefunctionality of wireless transmission and reception to operate inreception mode for monitoring possible paging from the service network.

However, due to the functionality of wireless transmission and receptionbeing turned off in the sleep mode, the mobile device may becomeunsynchronized with the service network in the OFF duration of the DRXoperation. That is, the timing drift and the frequency offset mayincrease too much for the mobile device to recover to be in-sync withthe service network. Generally, the mobile device relies onpilot/reference signals to re-synchronize with the service network, butthis inevitably results in an additional period of time for the mobiledevice to wake up from the sleep mode to receive the pilot/referencesignals from the service network. Not to mention that, in some mobilecommunication systems, e.g., the Long Term Evolution (LTE) system, thetime domain density of the pilot/reference signals is low, whichprolongs the additional period of time required for re-synchronization.

Alternatively, it is proposed for the mobile device to re-do a cellsearch to acquire the timing and frequency information of the servicenetwork, after detecting that it is out-of-sync with the servicenetwork. Nonetheless, it takes a long time to finish a cell search,which not only consumes more power, but also impacts user experience.

In another conventional practice, it is proposed for the mobile deviceto employ a better crystal to maintain the timing drift and thefrequency offset within an acceptable range when the mobile device is inthe sleep mode. However, a better crystal capable of providing such aneffect is expensive. As far as hardware cost is concerned, this practicehas a significant drawback.

Thus, it is desirable to have a more power-efficient and cost-conservingway of solving the synchronization issue raised during the DRXoperation.

BRIEF SUMMARY OF THE INVENTION

In one aspect of the invention, a mobile communication device isprovided. The mobile communication device comprises a Radio Frequency(RF) unit and a processing unit. The RF unit transmits and receiveswireless signals to and from a service network. The processing unitconfigures the RF unit to communicate with the service network in a DRXoperation, and uses a Physical Broadcast Channel (PBCH) forsynchronizing with the service network during the DRX operation.

In another aspect of the invention, a synchronization method executed bya mobile communication device is provided. The synchronization methodcomprises the steps of: communicating with a service network in a DRXoperation; and using a Physical Broadcast Channel (PBCH) forsynchronizing with the service network during the DRX operation.

Other aspects and features of the invention will become apparent tothose with ordinary skill in the art upon review of the followingdescriptions of specific embodiments of the mobile communication deviceand the synchronization method.

BRIEF DESCRIPTION OF DRAWINGS

The invention can be more fully understood by reading the subsequentdetailed description and examples with references made to theaccompanying drawings, wherein:

FIG. 1 is a schematic diagram of an exemplary DRX operation;

FIG. 2 is a block diagram of a mobile communication environmentaccording to an embodiment of the invention;

FIG. 3 is a block diagram illustrating the mobile communication device210 according to an embodiment of the invention;

FIG. 4 is a flow chart illustrating the synchronization method accordingto an embodiment of the invention;

FIG. 5 is a schematic diagram illustrating the RF status along withoccurrences of PBCH data and CRS according to the embodiment of FIG. 4;

FIG. 6 is a flow chart illustrating the synchronization method accordingto another embodiment of the invention;

FIG. 7 is a schematic diagram illustrating the RF status along withoccurrences of PBCH data and CRS according to the embodiment of FIG. 6;and

FIG. 8 is an exemplary diagram illustrating the synchronization usingPBCH according to an embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The following description is of the best-contemplated mode of carryingout the invention. This description is made for the purpose ofillustrating the general principles of the invention and should not betaken in a limiting sense. It should be understood that the embodimentsmay be realized in software, hardware, firmware, or any combinationthereof

FIG. 2 is a block diagram of a mobile communication environmentaccording to an embodiment of the invention. The mobile communicationenvironment 200 comprises a mobile communication device 210 and aservice network 220, wherein the mobile communication device 210 iswirelessly connected to the service network 220 for obtaining mobileservices, including voice and/or data services. The mobile communicationdevice 210 may be a feature phone, a smartphone, a panel PersonalComputer (PC), or any computing device supporting the wirelesstechnology utilized by the service network 220. The service network 220may be an LTE system, an LTE-Advanced (LTE-A) system, a Time-DivisionLTE (TD-LTE) system, or any future evolution of the LTE-based system.

To further clarify, the service network 220 comprises at least an accessnetwork 221 and a core network 222, wherein the access network 221 isresponsible for processing radio signals, terminating radio protocols,and connecting the mobile communication device 210 with the core network222, and the core network 222 is responsible for performing mobilitymanagement, network-side authentication, and interfaces with publicnetworks, e.g., the Internet. For example, if the service network 220 isan LTE/LTE-A/TD-LTE system, the access network 221 may be anEvolved-UTRAN (E-UTRAN) which includes at least an evolved NB (eNB), andthe core network 222 may be an Evolved Packet Core (EPC) which includesa Home Subscriber Server (HSS), Mobility Management Entity (MME),Serving Gateway (S-GW), and Packet Data Network Gateway (PDN-GW orP-GW).

FIG. 3 is a block diagram illustrating the mobile communication device210 according to an embodiment of the invention. The mobilecommunication device 210 comprises an antenna 10, a Radio Frequency (RF)unit 20, a Baseband chip 30, a display unit 40, an input device 50, anda storage unit 60, wherein the RF unit 20, the display unit 40, theinput device 50, and the storage unit 60 are coupled to the Basebandchip 30, and the antenna 10 is coupled to the RF unit 20. The Basebandchip 30 comprises a processing unit 31 for controlling the RF unit 20,sending a series of frame data (e.g. representing text messages,graphics, images or others) to the display unit 40, receiving signalsfrom the input device 50, and storing and retrieving data to and fromthe storage unit 60. The processing unit 31 may be a general purposeprocessor, a Micro-Processing Unit (MPU), a Micro Control Unit (MCU), aDigital Signal Processor (DSP), or others. Additionally, the Basebandchip 30 may further contain other hardware components to performbaseband signal processing, including Analog-to-Digital(ADC)/Digital-to-Analog (DAC) conversion, gain adjusting,modulation/demodulation, encoding/decoding, and so on. The RF unit 20may receive RF wireless signals via the antenna 10, convert the receivedRF wireless signals to baseband signals, which are processed by theBaseband chip 30, or receive baseband signals from the Baseband chip 30and convert the received baseband signals to RF wireless signals, whichare later transmitted via the antenna 10. The RF unit 20 may alsocontain multiple hardware devices to perform radio frequency conversion.For example, the RF unit 20 may comprise a mixer to multiply thebaseband signals with a carrier oscillated in the radio frequency of themobile communications system, wherein the radio frequency may be 900MHz, 2100 MHz, or 2.6 GHz utilized in LTE/LTE-A/TD-LTE technology, orothers depending on the wireless technology in use.

The display unit 40 may be a Liquid Crystal Display (LCD),Light-Emitting Diode (LED) display, or Electronic Paper Display (EPD),etc., for providing a display function. Alternatively, the display unit40 may further comprise one or more touch sensors disposed thereon orthereunder for sensing touches, contacts, or approximations of objects,such as fingers or styluses.

The input device 50 may comprise one or more buttons, a keyboard, amouse, a touch pad, a video camera, a microphone, and/or a speaker,etc., serving as the Man-Machine Interface (MMI) for interactions withusers.

The storage unit 60 may be a memory (e.g., Random Access Memory (RAM),Flash memory, or Non-Volatile Random Access Memory (NVRAM), etc.), or amagnetic storage device (e.g., magnetic tap or hard disk) an opticalstorage device (e.g., Compact Disc Read-Only Memory (CD-ROM)), or anycombination thereof for storing data, including instructions, programcode, and user data, etc.

Although not shown, the mobile communication device 210 may furthercomprise other functional units, such as a vibrator, a power supply, aGlobal Positioning System (GPS) unit for obtaining location information,or others, and the invention is not limited thereto.

Unlike conventional mobile communication devices, the mobilecommunication device 210 of the invention uses the Physical BroadcastChannel (PBCH), instead of the Downlink Control Channel (PDCCH) orPhysical Downlink Shared Channel (PDSCH), for synchronizing with theservice network 220 during the DRX operation. Specifically, the PBCH isused to carry system information, and the PDCCH/PDSCH is generally usedto carry Common Reference Signal (CRS) (also called pilot signal),wherein the time domain density of the Common Reference Signal (CRS) is1/3 to 1/4, i.e., there is 1 CRS in every 3 to 4 symbols, while the PBCHdata comes in 4 continuous symbols for every occurrence. Advantageously,the synchronization using the PBCH is more time-efficient and morepower-efficient than the synchronization using the PDCCH/PDSCH. Forexample, if 4 symbols are required for obtaining enough information forsynchronization, the mobile communication device 210 needs to wake upfor approximately 2 milliseconds (i.e., the period of time for 4continuous symbols) when using the PBCH for synchronization, andapproximately 6 to 8 milliseconds (i.e., the period of time for 12 to 16continuous symbols which contain about 4 CRS) when using the PDCCH/PDSCHfor synchronization.

FIG. 4 is a flow chart illustrating the synchronization method accordingto an embodiment of the invention. The synchronization method may beapplied to or executed by a mobile communication device, e.g., themobile communication device 210, and more specifically, thesynchronization method may be executed by a processing unit, e.g., theprocessing unit 31 of the Baseband chip 30, of the mobile communicationdevice. To begin, the mobile communication device successfully camps onor connects to a cell of a service network (step S410). In oneembodiment, the mobile communication device may be configured to operatein an idle mode, e.g., RRC_IDLE mode (i.e., the idle mode specified inthe Radio Resource Control (RRC) layer of the communication protocolutilized between the mobile communication device and the servicenetwork), after camping on the cell. In another embodiment, the mobilecommunication device may be configured to operate in a connected mode,e.g., RRC_CONNECTED mode (i.e., the connected mode specified in the RRClayer of the communication protocol utilized between the mobilecommunication device and the service network) after connecting to thecell.

It is noted that, the timing and frequency information of the cell isknown to the mobile communication device when it successfully camps onor connects to the cell. Next, the mobile communication device isconfigured to communicate with the service network in the DRX operation(step S420). That is, the processing unit configures the RF unit tocommunicate with the service network in the DRX operation. Specifically,the DRX operation may be configured by the service network via an RRCmessage, such as RRC CONNECTION SETUP message, RRC CONNECTIONRECONFIGURATION message, RRC CONNECTION REESTABLISHMENT message, orothers. Subsequently, the processing unit assigns the PBCH, which isclosest to and before the next ON duration of the DRX operation, to theRF unit (step S430), and then the mobile communication device enters thesleep mode in the OFF duration of the DRX operation (step S440).Specifically, when the mobile communication device is in the sleep mode,the RF unit is turned off or operates in a low-voltage state, and theprocessing unit operates in a low-voltage state as well.

Later, when the assigned PBCH comes, the mobile communication devicewakes up from the sleep mode to use the PBCH for synchronizing with theservice network during the DRX operation (step S450). Specifically, whenthe mobile communication device wakes up from the sleep mode, the RFunit is turned on or recovers to a high-voltage state for receiving PBCHdata, and the processing unit recovers to a high-voltage state as well.The synchronization with the cell of the service network is performed toresolve the timing drift and the frequency offset caused in the currentOFF duration of the DRX operation. Note that, the assigned PBCH comes inthe OFF duration of the DRX operation, so the mobile communicationdevice enters the sleep mode again after finishing the synchronizationwith the service network (step S460).

After that, when the current OFF duration ends and the next ON durationbegins, the mobile communication device wakes up from the sleep mode forreceiving PDCCH/PDSCH data and then demodulates and decodes thePDCCH/PDSCH data (step S470). After demodulating and decoding thePDCCH/PDSCH data, the mobile communication device determines whether ithas been paged by the service network (step S480). If the mobilecommunication device has not been paged by the service network in the ONduration, the synchronization method loops back to step S430. Otherwise,if the mobile communication device has been paged by the service networkin the ON duration, the synchronization method ends and the mobilecommunication device may proceed as instructed by the received pagingmessage (this part of detailed description is omitted herein as it isbeyond the scope of the invention).

FIG. 5 is a schematic diagram illustrating the RF status along withoccurrences of PBCH data and CRS according to the embodiment of FIG. 4.As shown in FIG. 5, the first row from the top denotes the occurrence ofPBCH data and CRS, and the second row denotes the RF status of themobile communication device (i.e., the status of the RF unit in themobile communication device). Although the PBCH data comes in eachsubframe 0, the assigned PBCH specifically refers to subframe 0 which isclosest to and before the next ON duration of the DRX operation.Particularly, the subframe 0 with the assigned PBCH is in the OFFduration of the DRX operation. Therefore, in the OFF duration, the RFstatus is initially set to “off”, and then set to “on” for receiving thePBCH data for synchronization with the service network. Specifically,for setting the RF unit, the RF status is set to “on” some time earlierthan the occurrence of the assigned PBCH, so that the RF unit may beready for receiving the PBCH data. As soon as the synchronization withthe service network is finished, the RF status is set to “off” againuntil the next ON duration. Similarly, for setting the RF unit, the RFstatus is set to “on” some time earlier than the next ON duration, sothat the RF unit may be ready for demodulating/decoding the receivedPDCCH/PDSCH data, including CRS.

FIG. 6 is a flow chart illustrating the synchronization method accordingto another embodiment of the invention. Similar to the embodiment ofFIG. 4, the synchronization method may be applied to or executed by amobile communication device, e.g., the mobile communication device 210,and more specifically, the synchronization method may be executed by aprocessing unit, e.g., the processing unit 31 of the Baseband chip 30,of the mobile communication device. To begin, the mobile communicationdevice successfully camps on or connects to a cell of a service network(step S610). In one embodiment, the mobile communication device may beconfigured to operate in an idle mode, e.g., RRC IDLE mode (i.e., theidle mode specified in the RRC layer of the communication protocolutilized between the mobile communication device and the servicenetwork), after camping on the cell. In another embodiment, the mobilecommunication device may be configured to operate in a connected mode,e.g., RRC CONNECTED mode (i.e., the connected mode specified in the RRClayer of the communication protocol utilized between the mobilecommunication device and the service network) after connecting to thecell.

It is noted that, the timing and frequency information of the cell isknown to the mobile communication device when it successfully camps onor connects to the cell. Next, the mobile communication device isconfigured to communicate with the service network in the DRX operation(step S620). That is, the processing unit configures the RF unit tocommunicate with the service network in the DRX operation. Specifically,the DRX operation may be configured by the service network via an RRCmessage, such as RRC CONNECTION SETUP message, RRC CONNECTIONRECONFIGURATION message, RRC CONNECTION REESTABLISHMENT message, orothers. Subsequently, the mobile communication device enters the sleepmode in the OFF duration of the DRX operation (step S630). Specifically,when the mobile communication device is in the sleep mode, the RF unitis turned off or operates in a low-voltage state, and the processingunit operates in a low-voltage state as well.

Later, when the current OFF duration ends and the ON duration begins,the mobile communication device wakes up from the sleep mode forreceiving PDCCH/PDSCH data and then demodulates and decodes thePDCCH/PDSCH data (step S640). After demodulating and decoding thePDCCH/PDSCH data, the mobile communication device determines whether ithas been paged by the service network (step S650). If the mobilecommunication device has been paged by the service network in the ONduration, the synchronization method ends and the mobile communicationdevice may proceed as instructed by the received paging message (thispart of detailed description is omitted herein as it is beyond the scopeof the invention). Otherwise, if the mobile communication device has notbeen paged by the service network in the ON duration, it is subsequentlydetermined whether the mobile communication device is out-of-sync withthe service network (i.e., an out-of-synchronization with the servicenetwork is detected) (step S660). If the mobile communication device isout-of-sync with the service network, the processing unit assigns thePBCH, which is closest to and after the ON duration, to the RF unit(step S670), and then the mobile communication device enters the sleepmode in the OFF duration (step S680). Otherwise, if the mobilecommunication device is not out-of-sync with the service network, thesynchronization method loops back to step S630.

In another embodiment of step S670, the processing unit may assign thePBCH, which is closest to and before the next ON duration, to the RFunit.

After that, when the assigned PBCH comes, the mobile communicationdevice wakes up from the sleep mode to use the PBCH for synchronizingwith the service network during the DRX operation (step S690), and thenthe synchronization method loops back to step S630. Specifically, whenthe mobile communication device wakes up from the sleep mode, the RFunit is turned on or recovers to a high-voltage state for receiving thePBCH data, and the processing unit recovers to a high-voltage state aswell. The synchronization with the cell of the service network isperformed to resolve the timing drift and the frequency offset caused inthe OFF duration(s) of the DRX operation.

FIG. 7 is a schematic diagram illustrating the RF status along withoccurrences of PBCH data and CRS according to the embodiment of FIG. 6.As shown in FIG. 7, the first row from the top denotes the occurrence ofPBCH data and CRS, and the second row denotes the RF status of themobile communication device (i.e., the status of the RF unit in themobile communication device). Before the ON duration of the DRXoperation, the RF status is initially set to “off”. Later, when the ONduration begins, the RF status is set to “on” for demodulating/decodingthe received PDCCH/PDSCH data, including CRS. Specifically, for settingthe RF unit, the RF status is set to “on” some time earlier than the ONduration, so that the RF unit may be ready for demodulating/decoding thereceived PDCCH/PDSCH data. When the ON duration ends, the RF status isset to “off” again until the occurrence of the assigned PBCH. As thePBCH data comes in each subframe 0, the assigned PBCH specificallyrefers to the subframe 0 which is closest to and after the denoted ONduration. Particularly, the subframe 0 with the assigned PBCH is in anOFF duration. That is, in the OFF duration, the RF status is initiallyset to “off”, and then set to “on” for receiving the PBCH data forsynchronization with the service network. Similarly, for setting the RFunit, the RF status is set to “on” some time earlier than the occurrenceof the assigned PBCH, so that the RF unit may be ready for receiving thePBCH data.

As to the synchronization using PBCH in steps S450 and S690, the generalidea is to rebuild the PBCH as pseudo-pilots for timing and frequencyadjustment. FIG. 8 is an exemplary diagram illustrating thesynchronization using PBCH according to an embodiment of the invention.Firstly, as shown in FIG. 8, the front end (i.e., the RF unit) isresponsible for receiving wireless signals and outputting theinformation of received wireless signals to the Fast Fourier Transform(FFT) function for converting frequency-domain data of the wirelesssignals to time-domain data, and/or vice versa, thereafter thetime-domain data of the wireless signals is temporarily stored inmemory. In the meantime, the PBCH is rebuilt as pseudo-pilots accordingto the PBCH data. Subsequently, the time-domain data stored in thememory is retrieved to De-pilot the pseudo-pilots and the results arefurther sent to estimate the timing drift and frequency offset.Specifically, for timing estimation, the timing drift may be calculatedbased on the changes of the channels, which are in neighboring carriers,in the frequency domain. Likewise, for frequency estimation, thefrequency offset may be calculated based on the changes of the channels,which are in neighboring symbols, in the time domain. After that, theestimated timing drift and frequency offset are fed back to the frontend for adjusting the timing and frequency in use, to be in-sync withthe service network. It is to be understood that, the diagram of FIG. 8is merely an illustrative example, and the invention is not limitedthereto. For instance, the FFT function may be pulled back to beperformed after the memory of the time-domain data, and a NumericalControlled Oscillator (NCO) may be introduced between the memory of thetime-domain data and the FFT function, which is responsible for creatinga synchronous (i.e., clocked), discrete-time, and discrete-valuedrepresentation of a waveform for the time-domain data. Alternatively, asthe timing drift and frequency offset are unknown, recursive selectionof a timing drift and a frequency offset from a plurality of possibletiming drifts and frequency offsets may be used fordemodulating/decoding the PBCH data, and if the demodulating/decoding issuccessful, the selected timing drift and frequency offset may beconsidered as correct.

While the invention has been described by way of example and in terms ofthe preferred embodiments, it is to be understood that the invention isnot limited to the disclosed embodiments. On the contrary, it isintended to cover various modifications and similar arrangements (aswould be apparent to those skilled in the art). Therefore, the scope ofthe appended claims should be accorded the broadest interpretation so asto encompass all such modifications and similar arrangements.

What is claimed is:
 1. A mobile communication device, comprising: aRadio Frequency (RF) unit, transmitting and receiving wireless signalsto and from a service network; and a processing unit, configuring the RFunit to communicate with the service network in a DiscontinuousReception (DRX) operation, and using a Physical Broadcast Channel (PBCH)for synchronizing with the service network during the DRX operation. 2.The mobile communication device of claim 1, wherein the synchronizationwith the service network is performed to resolve a timing drift and afrequency offset caused during the DRX operation.
 3. The mobilecommunication device of claim 1, wherein the synchronization with theservice network is performed in an OFF duration of the DRX operationbefore an ON duration of the DRX operation.
 4. The mobile communicationdevice of claim 3, wherein the OFF duration is the closest OFF durationto the ON duration.
 5. The mobile communication device of claim 3,wherein the RF unit is initially turned off in the OFF duration, and theprocessing unit further turns on the RF unit for the synchronization andturns off the RF unit after the synchronization until the next ONduration of the DRX operation.
 6. The mobile communication device ofclaim 1, wherein the synchronization with the service network isperformed in an OFF duration of the DRX operation after an ON durationof the DRX operation, when detecting an out-of-synchronization with the7. The mobile communication device of claim 6, wherein theout-of-synchronization is detected after finishing demodulating anddecoding Physical Downlink Control Channel (PDCCH) data or PhysicalDownlink Shared Channel (PDSCH) data received from the service networkin the ON duration.
 8. The mobile communication device of claim 6,wherein the OFF duration is the closest OFF duration to the ON duration,or the closest OFF duration to a next ON duration.
 9. The mobilecommunication device of claim 6, wherein the RF unit is initially turnedoff in the OFF duration, and the processing unit further turns on the RFunit for the synchronization and turns off the RF unit after thesynchronization until the next ON duration of the DRX operation.
 10. Asynchronization method executed by a mobile communication device,comprising: communicating with a service network in a DiscontinuousReception (DRX) operation; and using a Physical Broadcast Channel (PBCH)for synchronizing with the service network during the DRX operation. 11.The synchronization method of claim 10, wherein the synchronization withthe service network is performed to resolve a timing drift and afrequency offset caused during the DRX operation.
 12. Thesynchronization method of claim 10, wherein the synchronization with theservice network is performed in an OFF duration of the DRX operationbefore an ON
 13. The synchronization method of claim 12, wherein the OFFduration is the closest OFF duration to the ON duration.
 14. Thesynchronization method of claim 12, wherein a Radio Frequency (RF) unitof the mobile communication device is initially turned off in the OFFduration, and the synchronization method further comprises: turning onthe RF unit for the synchronization and turning off the RF unit afterthe synchronization until the next ON duration of the DRX operation. 15.The synchronization method of claim 10, wherein the synchronization withthe service network is performed in an OFF duration of the DRX operationafter an ON duration of the DRX operation, when detecting anout-of-synchronization with the service network in the ON duration. 16.The synchronization method of claim 15, wherein theout-of-synchronization is detected after finishing demodulating anddecoding Physical Downlink Control Channel (PDCCH) data or PhysicalDownlink Shared Channel (PDSCH) data received from the service networkin the ON duration.
 17. The synchronization method of claim 15, whereinthe OFF duration is the closest OFF duration to the ON duration, or theclosest OFF duration to a next ON duration.
 18. The synchronizationmethod of claim 15, wherein a Radio Frequency (RF) unit of the mobilecommunication device is initially turned off in the OFF duration, andthe synchronization method further comprises: turning on the RF unit forthe synchronization and turning off the RF unit after thesynchronization until the next ON duration of the DRX operation.